1. Field of the Invention
The present invention relates to a circuit arrangement for serial data transmission between a plurality of subscriber stations, comprising at least two data transmitters that have their output terminals connected to one and the same data bus which comprises two leads, respectively having a control input connected to a control arrangement and being controllable such that at most one of the data transmitters is activated at a given time, whereby the data transmitters, in their activated conditions, emit a voltage of a first polarity or a second polarity to the data bus dependent on the state of a data signal applied to the input, whereby the data transmitters have a higher internal resistance at the output in the passive condition than in the active condition, and whereby at least one data receiver is connected to the data bus, the data receiver, in the manner of a differential amplifier, assuming one of two prescribed signal states at its output given an input voltage and dependent on the polarity of the input voltage, and whereby the data bus is loaded with an effective load between the two leads at at least one location, the effective load being of such a nature that the voltage between the leads of the data bus has at least about the value zero given nonactive data transmitters.
2. Description of the Prior Art
A circuit arrangement of the type generally set forth above is already known from the periodical "Elektronik", 6/23 Mar. 1984, page 24 and 12/15 June 1984 pages 97 and 98.
The cited known circuit arrangement contains interface modules for serial transmission, each of which is composed of a data transmitter and a data receiver and is suitable for full-duplex or half-duplex operation. In half-duplex operation, the output of the data transmitter is directly connected to the input of the data receiver in each subscriber station. The subscriber stations can be a computer, for example, and peripheral devices connected thereto which are connected to a data bus. The data transmitter and the data receiver in each subscriber station can be driven into an active condition or into a passive condition by control signals.
Further, the Data Book of Texas Instruments, Interface Circuits, 1980 Edition, Page 388, discloses a circuit arrangement comprising interface modules in which both ends of the bus are terminated by a resistor having the value of the characteristic impedance. In relationship to the internal resistances at the output side of inactive data transmitters, such a resistor is so small that the voltage between the leads of the data bus has at least approximately the value zero.
Since, in a circuit arrangement of the type set forth above, the data transmitters, in the active condition, represent a low-impedance voltage source at their outputs whose voltage is dependent upon the logic level of the data signal applied to the respective input, only one data transmitter can be active at a given time.
To this end, the control signal that drives the data transmitter of a subscriber station into its active condition can be transmitted to all other subscribers and can be used at the other subscribers to make an activation impossible for the time in which the data bus is occupied. It is thereby obvious to transmit control signals via a control line or via a control bus which is provided in addition to the data bus.